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The origin of unequal bond lengths in the $\mathrm{\tilde{C}}$ $^1$B$_2$ state of SO$_2$: Signatures of high-lying potential energy surface crossings in the low-lying vibrational structure

The $\mathrm{\tilde{C}}$ $^1$B$_2$ state of SO$_2$ has a double-minimum potential in the antisymmetric stretch coordinate, such that the minimum energy geometry has nonequivalent SO bond lengths. The asymmetry in the potential energy surface is expressed as a staggering in the energy levels of the $ν_3'$ progression. We have recently made the first observation of low-lying levels with odd quanta of $v_3'$, which allows us--in the current work--to characterize the origins of the level staggering. Our work demonstrates the usefulness of low-lying vibrational level structure, where the character of the wavefunctions can be relatively easily understood, to extract information about dynamically important potential energy surface crossings that occur at much higher energy. The measured staggering pattern is consistent with a vibronic coupling model for the double-minimum, which involves direct coupling to the bound 2$^1$A$_1$ state and indirect coupling with the repulsive 3$^1$A$_1$ state. The degree of staggering in the $ν_3'$ levels increases with quanta of bending excitation, which is consistent with the approach along the $\mathrm{\tilde{C}}$ state potential energy surface to a conical intersection with the 2$^1$A$_1$ surface at a bond angle of $\sim$145$^{\circ}$.